It is well known that the resistivity of many conductive materials change with temperature. Resistivity of a positive temperature coefficient (PTC) conductive material increases as the temperature of the material increases. Many crystalline polymers, made electrically conductive by dispersing conductive fillers therein, exhibit this PTC effect. These polymers generally include polyolefins such as polyethylene, polypropylene and ethylene/propylene copolymers. At temperatures below a certain value, i.e., the critical or trip temperature, the polymer exhibits a relatively low, constant resistivity. However, as the temperature of the polymer increases beyond this point, the resistivity of the polymer sharply increases. Devices exhibiting PTC behavior have been used as overcurrent protection in electrical circuits comprising a power source and additional electrical components in series. Under normal operating conditions in the electrical circuit, the resistance of the load and the PTC device is such that relatively little current flows through the PTC device. Thus, the temperature of the device (due to I.sup.2 R heating) remains below the critical or trip temperature. If the load is short circuited or the circuit experiences a power surge, the current flowing through the PTC device increases and its temperature (due to I.sup.2 R heating) rises rapidly to its critical temperature. As a result, the resistance of the PTC device greatly increases. At this point, a great deal of power is dissipated in the PTC device. This power dissipation only occurs for a short period of time (fraction of a second), however, because the power dissipation will raise the temperature of the PTC device to a value where the resistance of the PTC device has become so high, that the original current is limited to a negligible value. This new current value is enough to maintain the PTC device at a new, high temperature/high resistance equilibrium point. This negligible or trickle through current value will not damage the electrical components which are connected in series with the PTC device. Thus, the PTC device acts as a form of a fuse, reducing the current flow through the short circuit load to a safe, low value when the PTC device is heated to the critical temperature range. Upon interrupting the current in the circuit, or removing the condition responsible for the short circuit (or power surge), the PTC device will cool down below its critical temperature to its normal operating, low resistance state. The effect is a resettable, electrical circuit protection device.
Polymer PTC electrical circuit protection devices are well known in the industry. Conventional polymer PTC electrical devices include a PTC element interposed between a pair of electrodes. The electrodes can be connected to a source of power, thus, causing electrical current to flow through the PTC element. The PTC element generally comprises a particulate conductive filler which is dispersed in an organic polymer. Materials previously used for electrodes include wire mesh or screen, solid and stranded wires, smooth and microrough metal foils, perforated metal sheets, expanded metal, and porous metals.
For example, U.S. Pat. No. 3,351,882 (Kohler et al.) discloses a resistive element composed of a polymer having conductive particles dispersed therein and electrodes of meshed construction embedded in the polymer. The mesh constructed electrodes disclosed in Kohler et al. are in the form of spaced-apart small wires, wire mesh or wire screening, and a perforated sheet of metal. Generally, electrodes of this type result in a PTC device with a high initial resistance even when the resistivity of the conductive polymer is low. In addition, the use of mesh electrodes with polymer PTC devices are susceptible to the formation of electrical stress concentrations, i.e., hot-spots, which can lead to subpar electrical performance, or even failure of the device. Moreover, conductive terminals which in turn are connected to a power source causing current to flow through the device are difficult to connect to mesh electrodes such as those disclosed in Kohler et al. Japanese Kokai No. 5-109502 discloses an electrical circuit protection device comprising a polymer PTC element and electrodes of a porous metal material. However, electrodes of this type also present difficulties when connecting conductive terminals to the porous electrodes, resulting in initially high resistant devices. U.S. Pat. Nos. 4,800,253 and 4,689,475 disclose electrical devices comprising one or more metal foil electrodes having a roughened surface which contacts a conductive polymer exhibiting PTC behavior. However, electrodes of this type restrict the type and structure of the housing or pakaging which the device may be used in.
The present invention solves these and other problems.